DH/DJ bike dimensioning

[Benj]

The use of top tube measurements in DH/DJ etc bike frame descriptions is probably historic but it's pretty useless.
Since most riding in these disciplines is out of the saddle surely the more useful measure is the horizontal distance between the bottom bracket and the top of the head tube (in combination with BB height, head angle and chainstay length this would much more effectivly convey the bike shape).
Because of the very slack seat tube angles often used in bikes with interupted seat tubes the measurement of these bikes in conventional terms (using top tube length) becomes loose and ambiguous. Using "effective" seat tube angles, BB heights and head tube heights it is possible to approximate this measurement but it's a bit vague and 'hides' the important information.
Bike reviews occasionally describe 'tight' or 'roomy' cock pits but it's not really a critical feature-certainly not as compared to the weight distribution and riding position determined by the BB-head tube dimention.
Looking through DH bike geometries Canfield Bros are the only ones I've found to supply this data.

[MUGEN]

I use top tube measurements all the time along with seat angle and head angle as a guide to how the front end will feel.
If you have a steep seat angled frame with a short top tube, it will feel similar to a slacker seat angle frame with long top tube when out of the seat. (to an extent ofcourse)
Some bikes like the norco duallies have the seat almost directly above the bottom bracket, making the seat angle almost vertical and the top tube being very short, confuses people as they think the bike is short when once your out of the saddle your weight is placed well behind the seat and there is ample clearance between knees and crowns.
I'm sure you can get the measurement you want with a bit of trigonometry.
Geometry means everything!